[Insight-developers] Use one algorithm or another in a filter, depending of the pixel type

[Gaetan Lehmann]

Jim,

I have used the following code to run the specialized version for some  
types:

   inline void AddPixel( const TInputPixel &p )
     {
     if( useVectorBasedAlgorithm() )
       { AddPixelVector( p ); }
     else
       { AddPixelMap( p ); }
     }

   inline bool useVectorBasedAlgorithm()
     {
     // bool, short and char are acceptable for vector based algorithm:  
they do not require
     // too much memory. Other types are not usable with that algorithm
     return typeid(TInputPixel) == typeid(unsigned char)
         || typeid(TInputPixel) == typeid(signed char)
         || typeid(TInputPixel) == typeid(unsigned short)
         || typeid(TInputPixel) == typeid(signed short)
         || typeid(TInputPixel) == typeid(bool);
     }

While it is less elegant than the overloaded implementation you proposed,  
it also let me easily specify a set of types usable with my specialized  
algorithm - I don't think it can be done easily with the overloaded  
implementation.
The bad point is that I had to do some static_cast in the code specialized  
for integer types to avoid warnings with real types - I guess that's not  
needed with overloaded implementation.

Anyway, that specialization give me an execution time about 4 times  
shorter better than with the general algorithm, which is very nice :-)

Gaetan

On Mon, 13 Mar 2006 15:17:14 +0100, Miller, James V (GE, Research)  
<millerjv at crd.ge.com> wrote:

> Gaetan,
>
> This can be done.  There is a certain pattern to follow that makes this  
> tractable.
> We call this a "dispatch" pattern.  Basically, you create a set of  
> overloaded functions.
> For instance, in itkMinMaxCurvatureFlowFunction, the dispatch pattern is  
> used to
> call either a 2D specific implementation, a 3D specific implementation,  
> or a general
> ND implementation. The same can be done for pixel types.  The pattern  
> creates
> a heirarchy of dispatch types so that the general case ends up calling an
> function overloaded on the root dispatch type.  Subclasses of the  
> dispatch
> type are used to provided overloaded functions for specific types.
>
> Here is the design from MinMaxCurvatureFlowFunction
>
>   struct DispatchBase {};
>   template<signed int VDimension>
>   struct Dispatch : DispatchBase {};
>  /** This method computes the threshold by averaging the intensity
>    *  in direction perpendicular to the image gradient. */
>   virtual PixelType ComputeThreshold( const Dispatch<2> &,
>                                       const NeighborhoodType &  
> neighborhood ) const;
>   virtual PixelType ComputeThreshold( const Dispatch<3> &,
>                                       const NeighborhoodType &  
> neighborhood ) const;
>   virtual PixelType ComputeThreshold( const DispatchBase &,
>                                       const NeighborhoodType &  
> neighborhood ) const;
>
> The appropriate function is called by the main entry point
>
>   threshold = this->ComputeThreshold( Dispatch<ImageDimension>(), it);
>
> This pattern will force only one of the overloaded functions to be  
> instantiated.  The
> same pattern can be used for pixel types (a DispatchBase class and a  
> Dispatch<short>, etc.).
>
> Jim
>
>
> -----Original Message-----
> From: insight-developers-bounces+millerjv=crd.ge.com at itk.org
> [mailto:insight-developers-bounces+millerjv=crd.ge.com at itk.org]On Behalf
> Of Gaetan Lehmann
> Sent: Sunday, March 12, 2006 10:21 AM
> To: insight-developers at itk.org
> Subject: [Insight-developers] Use one algorithm or another in a
> filter,depending of the pixel type
>
>
>
> Hi,
>
> Lots of algorithm can be implemented a lots more efficiently with some  
> pixels
> types - most of the time with char and short (signed or not).
> Is it possible automatically choose the algorithm used in a filter,  
> depending
> of the pixel type ?
> If yes, can it be done at build time ?
>
> Regards,
>
> Gaetan



-- 
Gaëtan Lehmann
Biologie du Développement et de la Reproduction
INRA de Jouy-en-Josas (France)
tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
http://voxel.jouy.inra.fr